Zirconium titanate (ZT)-based ceramic materials (in the form of solid solutions) have many unique properties such as high resistivity, high dielectric constant (thus providing high charge storage capacity), high permittivity at microwave frequencies, and excellent temperature stability of microwave properties. They have extremely wide applications such as in microwave telecommunications (as capacitors, dielectric resonators in filters and oscillators) and in catalysis as effective acid-base bi-functional catalysts and photocatalysts. In the form of thin films, they also find advanced applications in piezoelectric sensors, transducers, ultrasonic motors, hydrocarbon sensors, integrated microwave devices, refractory materials, high-temperature pigments, composites for high-temperature corrosive environments, and thin-film optics. In addition, ZT powders (crystalline ZrxT1-xO4) can be good precursor materials for synthesis of other valuable electroceramics such as lead zirconate titanate (PZT) or lanthanum doped PZT (PLZT) see Chen et at., “Hydrothermal Synthesis and Characterization of Crystalline ZrxT1-xO4 . . . ”, J. Mater. Sci., 1999, 1379–1383, 34; and Cerqueira et at., “Synthesis and Characterization of PLZT (9/65/35) by the Pechini Method and Partial Oxalate”, Mater. Lett., 1998, 166–171, 35, both references incorporated herein by reference). PZT is the most used electroceramic material in industrial applications as actuations and transducers.
As is true for many other materials, ultrafine-grained, high quality powders of ZT are in high demand. Fine powders are necessary precursors for making monolithic ceramics via casting as well as ceramic films via coating process. For such a binary (two metal elements) oxide system, compositional homogeneity and microstructure uniformity (low or no phase segregation) are very important. It is well known that powder characteristics such as particle size, shape, size distribution, agglomeration, crystallite size, chemical and phase composition, determine to a large extent, the microstructures developed during sintering and thus affect the properties of ceramic materials. On the other hand, it is necessary to use fine and single phase ZT powder to obtain fine and sinterable PZT powders by the partial oxalate method (Cerqueira et al., “Synthesis of Ultra-fine Crystalline ZrxTi1-xO4 Powder by Polymeric Precursor Method”, Mater. Lett., 1995, 181–185, 22).
In order to obtain high purity and better homogeneity materials, various chemical solution synthesis methods have been developed as alternatives to the conventional solid-state-reaction route, which normally requires high temperature (1200° C.–1700° C.) over a prolonged period for the homogeneous materials synthesis from the mixed crystalline ceramic oxides ZrO2 and TiO2 and further requires post treatment such as energy-intensive grinding/milling procedures for powder formation. Still, this usually leads to inhomogeneous, coarse, and multiphase powders of poor purity. Amorphous precipitates or gels (called precursors) are usually produced through chemical solution routes, which are characterized by their unparalleled ability to generate ultrafine, high purity and stoichiometric ceramic powders at low processing temperature. Pure oxide materials can be obtained by thermal processes of dehydration and crystallization of precursor precipitates, gels or particles. Several major wet chemical routes include: sol-gel processes; synthesis from metallorganic salts; chemical precipitation and coprecipitation of metal salts from aqueous solutions; mixed-cation oxides via thermal decomposition of polymeric precursors wherein powders produced by this method are usually irregular in shape, strongly agglomerated because of the thermal decomposition step, and widely distributed in size; and high-energy ball-milling.
Among the aforementioned synthesis routes, few could produce ultrafine powders containing aggregation-free, monodispersed, microsphere particles. Some routes such as the classical sol-gel process and some polymeric precursor methods involve the use of expensive metal alkoxide salt(s) or commercially unavailable metallorganic salts. Most routes, such as gel-forming via precipitation or polymeric precursor methods, still require the undesirable procedure of grinding dried gels into powders with no control of particle shape. Irregular shaped powdered ceramic particles are both difficult to handle and use on an industrial scale and thus spherical particles are preferred for many distinct advantages. The submicron, spherical particles reported by Hirano et al. at “Chemical Processing and Microwave Characteristics . . . ”, J. Am. Ceram. Soc., 74, 1320–24, (1991), obtained by controlled hydrolysis of metal alkoxides, are agglomerated. Bhattacharya et al. “Sol Gel Preparation, Structure and Thermal Stability of Crystalline Zirconium Titanate Microspheres”, J. Mater. Sci. 31, 267–271, (1996), produced dispersed, sphere-shaped particles; however, the size of the reported “microspheres” was in the range of 15–50 μm, which is quite large for applications that require ultrafine particles (i.e., submicron to a few micrometers in diameter).